The present invention relates to a fuel cell that uses pure hydrogen or hydrogen reformed from methanol or fossil fuels as a fuel or that directly uses liquid fuel such as methanol, ethanol or dimethyl ether, together with air or oxygen as an oxidant. The present invention particularly pertains to a fuel cell comprising a solid polymer electrolyte.
A conventional electrode for polymer electrolyte fuel cells is composed of a catalyst layer which comes in contact with a polymer electrolyte membrane and a gas diffusion layer provided on the outer surface of the catalyst layer. The gas diffusion layer has three major functions. The first one is a function of diffusing a reaction gas for evenly supplying the reaction gas such as a fuel gas or an oxidant gas to the catalyst of the catalyst layer from a gas flow channel formed on the outer side of the gas diffusion layer. The second is a function of promptly discharging water produced by the reaction at the catalyst layer into the gas flow channel. The third is a function of conducting electrons required or generated by the reaction. Therefore, the gas diffusion layer must have high properties in terms of reaction gas permeability, water permeability and electronic conductivity.
Conventionally, the gas diffusion layer has a porous structure to have the gas permeability, and contains a water-repellent polymer, such as fluorocarbon resin, which is dispersed for suppressing water clogging (flooding), to have the water permeability. Also, the gas diffusion layer is conventionally made of an electron conductive material, such as carbon fiber, metal fiber or carbon fine powder, to have the electronic conductivity.
As a material having the three functions, carbon paper is conventionally used for the gas diffusion layer. However, the use of carbon paper for the gas diffusion layer of a fuel cell invites flooding when the relative humidities of the fuel gas and the oxidant gas become 98% or higher, so that the voltage of the fuel cell is lowered, resulting in unstable operation. This is due to the high gas diffusibility of carbon paper which increases the proportion of an underflow of the reaction gas. The underflow of the reaction gas is a flow of the reaction gas through the gas diffusion layer under the rib formed by the gas flow channel of a separator plate, and when the proportion of the underflow is increased, the pressure loss of the reaction gas is reduced, thereby impairing the water permeability. Thus, if the flow rate of the reaction gas is increased to heighten the pressure loss of the reaction gas, stable operation becomes possible, but this also causes a decrease in efficiency of the fuel cell.
On the other hand, a fuel cell using carbon fiber non-woven fabric for the gas diffusion layer is capable of stable operation even when the relative humidities of the fuel gas and the oxidant gas are 98% or higher. It is noted, however, that the voltage of such a fuel cell is lower than that of the fuel cell using carbon paper. The fuel cell using carbon fiber non-woven fabric is resistant to the flooding even at high degrees of humidification of the supplied gases because of the following reason. Due to the clamping pressure of the assembled fuel cell stack, the carbon fiber non-woven fabric is compressed as a whole, so that the gas permeability of the carbon fiber non-woven fabric is lowered, resulting in a decreased proportion of the underflow and a high pressure loss. However, the lowered gas permeability also causes a decrease in discharge performance of the fuel cell.
The present invention solves these problems and aims to provide an electrode for use in fuel cells which ensures gas diffusibility and excessive water permeability of a gas diffusion layer.
The present invention also aims to provide a fuel cell having high discharge performance and stability particularly in power generation under highly humidified conditions by reducing the proportion of an underflow while ensuring the porosity of the gas diffusion layer necessary for gas diffusion.